1. Field of the Invention
The present invention relates to an improved process of nitrilo tris-(methylene phosphonic acid) and more particularly to a process of producing substantially pure, crystalline nitrilo tris-(methylene phosphonic acid).
2. Description of the Prior Art
A number of processes of producing nitrilo tris-(methylene phosphonic acid) are known. Thus said compound is obtained according to German published application No. 1,214,229 by reacting ammonium chloride, formaldehyde, and phosphorous acid in the proportion 1:3:3. The yield of pure, solid compound is not given. It has merely been determined by means of the nuclear magnetic resonance spectrum (NMR-spectrum) of the reaction solution that 85% of the orthophosphorous acid have been reacted to form the desired N-C-P linkage.
Subsequently it has been reported in German published application No. 1,259,337 that, on determining the yield by NMR-analysis of the reaction solution, byproducts are included and that pure nitrilo tris-(methylene phosphonic acid) precipitates from the reaction solution obtained according to German published application No. 1,214,229 in crystalline form only very slowly and within several days. The yield does not exceed about 46%. It is furthermore stated in said published application No. 1,259,337 that the yield can be increased to 86% by reacting an excess of phosphorous acid between 20 and 80% and preferably between 30 and 50%. However, even when proceeding in this manner, crystalline nitrilo tris-(methylene phosphonic acid) precipitates only very slowly, namely at 20.degree. C. within 16 hours.
British Pat. No. 1,142,294 discloses the preparation of nitrilo tris-(methylene phosphonic acid) by reacting ammonium chloride, 37% aqueous formaldehyde solution, and phosphorus trichloride. The yield as given in said British Pat. No. 1,142,294 was also determined by analyzing the reaction solution. Isolation of the crystalline compound has not been described in the examples.